An optical disc apparatus for the next generation is required to operate with a large capacity of information and the high transfer rate information. For this purpose, an objective lens for high resolution recording is used for an optical head, and is required to be controlled with a high speed and the high precision. Therefore, an objective lens actuator to be moved in focus and track directions must be small in size and light in weight, and must generate a driving force.
A first conventional actuator for an optical head comprises a lens holder which holds an objective lens, a focus drive coil provided around an outer periphery of the lens holder, tracking drive coils which are flat to be provided on the focus drive coil, an actuator base having a shaft for supporting the lens holder to be moved along the shaft and rotated around the shaft, a pair of magnets provided on opposite sides of the shaft on the actuator base, and a damper spring provided on the actuator base to resiliently energize the lens holder positioning the neutral position by engaging with the lens holder, as disclosed in the Japanese Patent Kokai No. 1-178134.
In operation, driving adjusting current in tracking direction flows in a direction through the tracking drive coils, so that a tracking adjusting magnetic field is generated to link with a magnetic field of the magnets. As a result, the lens holder is rotated against the force of the damper spring around the shaft on the actuator base In a direction determined by the direction of the tracking driving current.
On the other hand, a focus adjusting current flows in a direction through the focus drive coil, so that a focus adjusting magnetic field is generated to link with the magnetic field of the magnets. As a result, the lens holder is moved up and down in direction, against the rebound force of the damper spring dependent on the direction of the focus adjusting current.
Thus, the focus and tracking adjustments of the objective lens are carried out in accordance with focus and tracking moving forces generated by the magneto-electrical forces between the generated magnetic flux and the magnetic field.
A second conventional actuator for an optical head comprises a lens holder for holding an objective lens, a pair of parallel plate springs for resiliently supporting the lens holder to move in the focus adjusting direction, a hinge for allowing the lens holder to rotate around a hinged point, a focus drive coil surrounding the plate springs, tracking drive coils which are flat to be provided on the focus drive coil, and magnets provided on opposite sides of the focus and tracking drive coils, as disclosed in the Japanese Patent Kokai No. 4-38725. The detail of the structure and operation will be later explained in FIGS. 2A and 2B.
In the first conventional actuator for an optical head, however, there are disadvantages in that the efficiency of electromagnetic force conversion is low in the tracking drive coils, because a drive force is generated only on one side of each tracking drive coil, when the tracking drive coils are moved to adjust the objective lens in the tracking direction, and that the motion amount of the objective lens relative to a driving current is non-linear, because a deviation force acts between a bearing of the lens holder and the shaft of the actuator base according to the damper spring layout. Especially, this tendency is remarkable, when an objective lens having a large focus, depth is used. This results in the difficulty in providing a predetermined precision control in the focus direction.
Further, there are disadvantages in the second conventional actuator for an optical head in that the design of providing a fitting tolerance between the hinge made of nylon and the lens holder made of polyphenylene sulfide is difficult to result in hinge deformation, that the molding of the hinge is difficult due to poor fluidity of nylon, because a hinge portion is as thin as 20 to 30 .mu.m, and the temperature characteristics of nylon are not so well, and that a weight for moving members is large due to counter balances; spacial limitation in arranging a magnetic circuit and the focus and tracking drive coils for the structural reason, therefore, it is difficult to increase a drive sensitivity, especially, that of the tracking drive coils, and to stabilize the temperature characteristics of the focus and tracking drive coils.